Direct liquid-writing instruments such as ballpoint and fountain pens using a water-ink have, as a component, an ink-retaining part in a bellows-like shape made of a thermoplastic resin such as ABS resin. The ink-retaining part is required to have a good wettability of ink. If the wettability is poor, ink-leakage occurs from the pen-tip during use, which raises problems of getting users' hands dirty or of soiling papers.
JP-A 8-156481 discloses a method for forming a thin film of a surfactant or of a hydrophilic substance on the surface of the ink-retaining member 2, a method for forming an irregular profile on the surface of the ink-retaining member 2 caused by etching treatment, deposits, and the like, and other methods.
JP-A 9-315079 discloses an etching treatment for improving the wettability of a resin part (a pen core 14 including the ink-retaining portion 14b) of the direct liquid-writing instrument, using an etching liquid composed of sulfuric acid as the base compound and, further, of chromic anhydride and phosphoric acid. As described also in JP-A 9-315079, however, the etching liquid has a problem of a strong toxicity.
JP-A 11-268470 discloses a method for applying chemical treatment using an acid such as hydrochloric acid, nitric acid, or a chromic acid mixture, and using an alkali, and the like for improving wettability of resin part (such as a collector (ink-retaining body) 2b which is an adjustor for adjusting the internal pressure of the writing instrument using a plurality of blade-shaped grooves) of the direct liquid-writing instrument.
JP-A 2005-88289 shows a direct liquid-writing instrument having an ink-retaining part formed by a thermoplastic resin having a permanent antistatic, but is not sufficient in prevention of ink-leakage.